Cell size overrides ganglion mother cell fate specification in Drosophila

The size of cells varies enormously in biology and how cell size and function are linked are fundamental questions. Asymmetrically dividing Drosophila neural stem cells, also called neuroblasts, produce a large neuroblast and a smaller ganglion mother cell (GMC) at each division. However, why GMCs need to be smaller than neuroblasts has largely remained unexplained. Here, we developed acute and reversible methods to alter daughter-cell size during a single neuroblast division while preserving cortical polarity and asymmetric inheritance of fate determinants. We identify atypical protein kinase C (aPKC) as a regulator of size asymmetry and show that transient and partial co-inhibition of aPKC and Rho kinase (Drok in Drosophila) generates daughters spanning a size range, including size-wise near-symmetric sibling cells while polarity is preserved. We find that above a certain size threshold daughter cells retain neuroblast-like properties despite inheriting GMC determinants: they divide repeatedly, exhibit neuroblast-like cell-cycle timing, and undergo size-wise asymmetric division. Smaller daughters below this threshold instead follow a GMC-like trajectory. Thus, in Drosophila type I neuroblasts, daughter-cell size can act as a regulator of lineage behaviour, overriding inherited fate cues in sufficiently large daughter cells.